This invention relates to a downcomer apparatus and to a vapor-liquid contact apparatus provided with the downcomer apparatus, and preferably to a chemical processing vapor-liquid contact apparatus in which a vessel contains a plurality of substantially horizontal trays which support a vapor-liquid mixture. Liquid is introduced at the upper end of the vessel and it flows down from tray-to-tray, via a plurality of stepped downcomer apparatus, and the trays are apertured to provide bubbling areas through which ascending vapors can rise to contact liquid and/or vapor-liquid mixtures which are supported on and flowing across the trays.
It has been recognized in the art that the performance of a contact tray apparatus can be enhanced if the liquid flow on the tray is uniform in the respect that the flow in the lateral areas of the tray is substantially the same as the flow along the central flow axis thereof. Heretofore, shaped downcomer tips, directional vapor outlets and other means have been utilized for this purpose.
It has also been recognized in the art that the performance of a contact tray apparatus can be enhanced in some situations if the inside wall of the downcomer is sloped from vertical in order to create a decreasing downcomer cross-sectional area from the top to bottom. This design effectively maximizes the downcomer inlet opening and minimizes the area occupied by the bottom of the downcomer, thus maximizing the bubbling area for additional vapor flow. As a downcomer on the side portion of a tray is sloped to decrease the cross-sectional area, the length of the chordal liquid release decreases proportionally. Typically, the degree of downcomer sloping is often limited in order to maintain a chordal liquid release length no less than 60% of the tower diameter in order to ensure adequate liquid distribution onto the tray.
There has also been utilized in the art truncated downcomers which are trough like and contain an integral floor that is elevated above the deck of the tray below and extends outward from a supporting downcomer wall. The truncated downcomer floor has apertures designed to control the liquid flow to the tray below and also forms a liquid seal to prevent vapors from flowing upward through the downcomer. The liquid release from a truncated downcomer is vertically downward as opposed to a conventional downcomer which releases the liquid horizontally onto the tray as a moving fluid body past a vertical spacing defined by a downcomer lower edge. The elevated apertured floors of a truncated downcomer are designed to physically separate the downcomer from the tray deck in order to allow the deck area underneath the downcomer to be perforated for vapor flow thus increasing the effective bubbling area of the tray. The elevated apertured floor also provides a downcomer discharge location separate from the hydraulic head and turbulence of the frothy mixture on the tray below. However truncated downcomer designs have some inherent limitations. The design requires a dynamic seal where the pressure drop of the liquid leaving the floor apertures prevents vapor from flowing up the downcomer from the tray below adversely affecting the tray performance. The truncated downcomer, by definition, shortens the vertical downcomer length. With some liquid/gas mixtures, the truncated design may not provide sufficient downcomer length causing the fluid in the downcomer to back up onto the tray above, thus limiting the tray capacity. The floor orifices in truncated downcomers are susceptible to plugging from particulate matter in systems where solids are present in the operating fluids. Also, the downward release of liquid from the apertured floor tends to cause the liquid to be released unevenly onto the tray deck below.
When the target area of an element of liquid downflow from a truncated downcomer is near an obstruction such as the inner wall of a vessel, the liquid capillary wave celerity emanating radially from the liquid impact point in the target area will strike and rebound from the vessel wall. Due to the concave curvature of the wall of a cylindrical vessel, some of the rebounding capillary wave liquid will be directed toward the central flow axis of the tray, thus causing a focusing effect which results in a higher flow rate at the central flow axis of the tray than at the sides thereof. This effect creates a liquid peak at the central flow axis and causes liquid recirculation eddy tendencies along the sides of the central flow. Both of these occurrences tend to reduce tray mass transfer efficiency and decrease the effective capacity of the tray. Heretofore, specialized floor orifice patterns and directional vapor outlets on the tray decks other means have been utilized to mitigate this effect.
U.S. Pat. No. 6,003,847 describes a prior art downcomer embodiment that utilizes a downcomer with a highly sloped, semi-conical wall with an outlet formed by a lower edge of the semi-conical wall and the inner surface of the tower wall to control the release of the liquid traveling between that lower edge and the tower wall which liquid then travels down to the tray below. The outlet opening defined by the lower edge of the semi-conical wall and tower wall has a central outlet portion and outer outlet portions with the outer portions increased in size as compared to the center portion. This arrangement is described as providing more liquid flow through the opposing end portions than through the center of the downcomer outlet and also is indicated as being considered to provide a more uniform flow across the tray. In conjunction with the above noted outlet opening, U.S. Pat. No. 6,003,847 describes the use of a multi-chordal inlet weir offset horizontally inward on the tray receiving liquid from the flow controlling semi-conical outlet above to control the liquid flow traveling out onto the tray deck. From this device, the volumetric flow will be proportional to the length of the slot or inlet weir. The concave weir, by definition, releases a disproportionate amount of liquid to the center of the flow path and also directs the liquid towards a focal point located on the flow path centerline, thus creating uneven liquid distribution on the tray deck. Heretofore, devices of this type have used directional vapor apertures on the tray deck to redistribute the liquid. However, these devices have only limited effectiveness within a limited range of equipment operation.
Also in prior art, sloped or small downcomer tray designs in large diameters may often do not have sufficient mechanical strength and require separate trusses or beams that would likely impede the flow of the gas-liquid mixture on the tray deck and limit capacity.
Under the present invention, a downcomer is provided that is designed to maximize the active bubbling area available and maintain the maximum downcomer length available for separation of the vapor-liquid mixture in the downcomer, while also providing uniform liquid flow distribution at the inlet edge of the active bubbling area. The downcomer of the present invention is also designed to enhance tray and downcomer structural support.
By providing a more uniform liquid flow distribution at the inlet edge of the active bubbling area, the present invention is also directed at avoiding the aforementioned problem of having the liquid flow non-uniform due to a greater amount of centralized flow and recirculation eddy tendencies along the sides of the central flow.
Thus, according to the present invention, the flow of fluid into the bubbling area of a tray is made more uniform across the width of the bubbling area by providing a novel shape and location of a liquid passageway defined by a downcomer outlet which feeds liquid evenly onto the tray. In a preferred embodiment, for example, a downcomer is provided that has a unique chordal shape and means for controlling the vertical column height of liquid flow traveling between the downcomer lower edge and tray below, which arrangement helps avoid the problem of having the liquid flow non-uniform due to a greater amount of centralized flow and recirculation eddy tendencies along the sides of the central flow. The design of the present invention is therefore directed toward providing increased capacity, higher efficiency, a greater operating range as well as minimization of the risk of fouling or plugging of holes or other smaller multiple openings at the downcomer outlet. The design also improves structural support efficiency. Furthermore, by providing for easy adjustability of the components which define the novel downcomer outlet opening, the apparatus can be adjusted upon installation to accommodate tower out-of-roundness and assure symmetrical liquid outflow to achieve optimum performance.
The mechanical design of this invention allows for a stronger, simpler, and less intrusive support structure for the tray above. This is especially important with larger diameter designs with moderate to low tray spacings.
The present invention is also directed at avoiding the performance degrading effect of the downcomer focusing liquid towards the liquid flow path centerline through use of a stepped downcomer with an elongated multi-chordal downcomer outlet opening featuring a continuous slot or slots directing liquid flow parallel to the tray central flow axis, with a maximum effective chordal downcomer escape length.
This invention is thus based in part on the recognition that there are inherent deficiencies in the liquid distribution at the upstream portion of the bubbling areas of the trays in the apparatus described in the prior art, particularly due to the configurations and arrangements of the apertures in the truncated downcomer floors, in the configuration of continuous arcuate slots and weirs, and in the arrangement and other design features of the prior art downcomers. The configuration of the present invention features a step downcomer that provides a side downcomer escape with the maximum chordal downcomer escape length spacing which can be designed to properly proportion the liquid flow evenly onto the tray bubbling area. The configuration of the present invention further provides the maximum potential bubbling area while distributing liquid evenly onto the bubbling area in a direction parallel to the central flow axis to resist retrograde flow. The design of the present invention is also such that the downcomer features open areas at the radial outward step area and the opposing ends of the horizontal portion of the step of the downcomer which permit additional liquid downflow at the ends of the downcomer""s cross-sectional area. In a preferred embodiment, these openings extend to the tower wall; therefore, troublesome horizontal downcomer-to-tower wall attachments used with conventional stepped downcomers are not needed thus reducing design and installation complexity.
In addition, the downcomer design of the present invention includes a step section replacing the lower portion of a vertical chordal downcomer. The stepped section of the downcomer of the present invention preferably includes a step wall arrangement that has a major vertical component and extends in a downward direction (i.e., directly vertically down or in an oblique relationship to a vertical plane) from an outer periphery of the generally horizontal downcomer step panel. In a preferred arrangement the downcomer step wall features a plurality of wall panels that are preferably integral or bolted together and, when combined with the vertical chordal downcomer panel above the step platform, define a radial inward wall region in the downcomer that is free of holes or apertures. The opposing end panels of the multi-panel step wall are hereafter referred to as tiplets. The tiplets enclose the end portions of the downcomer step openings which are radially external such that the panels prevent vapor bypassing into the downcomer. The tiplets are positioned to re-direct the additional liquid within the downcomer towards the ends of the downcomer escape. For many usages of the present invention the tiplets are vertical. For high pressure or other cases requiring a greater difference in the downcomer cross-sectional area at the top and bottom, the tiplets can be inclined radially outward at the bottom to reduce the enclosed area at the bottom of the downcomer. For some embodiments, particularly larger diameter towers, it is helpful to have chordal extensions that extend out from respective vertical edge end regions of the tiplets (opposite the vertical edge end region of the tiplets to which the central chordal wall panel is attached) toward the tower wall and into a supporting relationship therewith.
An additional preferred feature of the present invention is that, at the weir, the vertical upper downcomer truss panel supports the downstream ends of a tray deck panel arrangement. Also, for small columns the tiplets and central chordal step wall panel can be integrated into one piece and will not require a separate set of clamping bars for attachment to the tower. That is, rather than a separate set of clamping bars, the entire set of step wall panels are supported by the top downcomer chordal panel that forms the major vertical supporting truss.
For large columns with low tray spacings, relatively short, vertical support members such as vertical channel members, preferably oriented parallel to the central flow axis, or tiplet extensions extending down to the tray below can be added to support the truss panels from the deck trusses on the tray below.
The lower edge of the tiplets, the central chordal step wall panel, and the extensions extending out from the interior of the tiplets and preferably into engagement with corresponding tower wall supports, provide the means for controlling the vertical liquid flow through column height with respect to the underlying tray. The spacing between the lower edge of each of these components of the stepped downcomer can also be adjusted either in unison or relative to each other to achieve a desired spacing between the lower downcomer panel edges and the tray below. The portions of horizontal cross-sectional outlet areas provided at the center and extremities at the bottom of the downcomer can be independently adjusted to provide optimum liquid flow distribution as such modifications alter the position of the lower edge of one or more of the aforementioned lower edges and hence the location of where the vertical flow controlling downcomer edge is located relative to the tray below. With respect to truncated downcomers where the horizontal cross sectional opening is controlling and not the vertical flow through spacing, larger areas at the extremities at the bottom of the downcomer have been found to be beneficial in truncated downcomer configuration testing. In this regard, reference is made to copending U.S. application Ser. No. 09/413,885 filed on Oct. 7, 1999 (now U.S. Pat. No. 6,250,611) featuring a floor platform which provides desirable downcomer flow through liquid handling capacity attributes. The peripheral location and configuration of the step platform can also be set, such that either though use of vertical wall panels alone or in conjunction with sloping tiplets, for example, a desired liquid vertical flow through passageway and liquid handling arrangement can be achieved. Other liquid handling features like sweepback weirs at the top of segmental side downcomers can be used to further enhance downcomer liquid handling capacity.
According to one principal feature of the present invention, a vapor-liquid contact apparatus comprises a vessel, and a plurality of vertically spaced horizontal trays in the vessel for supporting a vapor-liquid mixture. Each of the trays has a bubbling area, a liquid infeed area at an upstream end of the bubbling area, and an exit opening at a downstream end of the bubbling area. The bubbling area has a central flow axis which leads from the liquid infeed area to the exit opening, and apertures which permit ascending vapors to flow up through the tray and into a vapor-liquid mixture on the tray. A downcomer is provided for receiving the vapor-liquid mixture from the exit of the tray above and for carrying and uniformly distributing liquid to the tray below. The downcomer has an upper portion with a cross-section which is surrounded by an inner edge and a concave outer edge. These edges are perpendicular with respect to the central flow axis (e.g., perpendicular to the preferably straight lined inner edge and perpendicular to the tangent of the concave outer edge at the central flow axis). The cross-section of the downcomer has a centerline which is the same as the central flow axis of the bubbling area. A step is provided in the above-mentioned cross-section and at an intermediate elevation of the downcomer""s vertical height to reduce the size of the downcomer cross-sectional area adjacent to the bubbling area.
From the outer periphery of the intermediate step there extends downwardly a step wall extending from, and conforming in shape (at least generally) to, the radially outer peripheral edge of the step. The downcomer step wall defines the cross-sectional area of the bottom portion of the downcomer. The opposing ends of the step wall are referred to as tiplets. The lower or bottom edge of the step wall preferably has a cross sectional configuration with a shape common to that of the upper region either positioned at a common radial position (e.g., vertical tiplets defining similar opening spacing between the upper and lower regions of the tiplet portions with respect to the radially outward positioned concave outer edge of the downcomer) or at an oblique orientation wherein, from the step level downward, the bottom of the downcomer has decreasing horizontal, cross-sectional areas). For instance, outwardly sloping tiplets (preferably generally conforming in shape to the upper edge portion except for the sloping) are provided that are sloped radially outward in their vertical extension to the lower edge of the tiplet regions. In this way, there is a decrease in the area of the downcomer cross-sectional passageway going from the intermediate step down to the downcomer""s lower edge.
From a horizontal cross-section standpoint, the downcomer outlet opening is comprised of an elongated downcomer outlet slot or series of slot sections, the width being perpendicular to the lower edge of the lower downcomer step wall panels and the length being parallel with respect to the lower edge of the lower downcomer step wall panels. The slot width at the centerline of the horizontal cross-section at the level of the outlet opening is preferably no greater than the slot width at locations which are spaced from the centerline.
Also, by providing the desired initial chordal length for the central panel and the desired angle in the tiplets (in their extension from the central choral panel out into contact with the extensions extending out into a support relationship with the tower), the relative width of the extensions (preferably chordal extensions extending parallel with the central step wall panel), can be set to achieve high performance in vapor-liquid contact across the tray. In addition to the central chordal panel""s lower edge extending perpendicular to the central flow axis, the two extensions extending from the tiplets also present lower edges that are perpendicular to the central flow axis and the length of that lower edging can be set to take advantage of the relatively large volume of liquid flow traveling to opposite sides of the step platform and down the tiplets. This achieves a desirable high flow volume in a direction parallel to the central flow axis so as to avoid the problem of having the liquid flow non-uniform due to a greater amount of centralized flow and recirculation eddy tendencies along the sides of the central flow. The relative percentage of the length of downcomer exit edging extending perpendicular to the central flow axis and that which forms the tiplets and is obliquely arranged to the flow axis (and which determines or generally determines the step platform""s configuration), can be readily set under the design of the present invention to best accommodate the intended use of the downcomer. Having a large percentage of lower downcomer exit edging extending perpendicular to the central flow axis also enhances the versatility of the type of vapor aperture valves that can be utilized in the trays. For example, vapor aperture valves having valves with side gas outlets and blocked upstream and downstream ends can be used in these areas with little concern for liquid introduction into the side outlets.
According to another main feature of the invention, a vapor-liquid contact apparatus comprises a vessel, and a plurality of vertically spaced horizontal trays in the vessel for supporting a vapor-liquid mixture. Each of the trays has a bubbling area, a liquid infeed area at an upstream end of the bubbling area, and an exit opening at a downstream end of the bubbling area. The bubbling area has a central flow axis which leads from the liquid infeed area to the exit opening, and apertures which permit ascending vapors to flow up through the tray and into a vapor-liquid mixture on the tray. A downcomer is provided for receiving the vapor-liquid mixture from the tray and for carrying liquid to another tray. The downcomer has an upper portion located at the exit opening of the tray. This upper portion defines an upper opening of the downcomer with respect to a horizontal cross-section that preferably features a straight line inner boundary and a preferably concave radially outer boundary. The downcomer also has an intermediate height region that includes an intermediate generally horizontal step platform having an inner straight edge parallel to and on a common vertical plane with the inner boundary chordal edge of the top of the downcomer opening and an outer chordal edge that, in conjunction with the outward concave boundary of the downcomer, defines a cross-sectional opening region. The chordal sides of the generally horizontal step platform are symmetrically truncated and shorter than the full downcomer chord length providing two end opening regions. Extending downwardly from the step platform symmetrically at each end is a tiplet portion which, along with the other step wall panel(s) define a lower region of the downcomer. The cross-section of the downcomer (including that for the upper portion, intermediate step portion and lower tiplet portion) has a centerline which is identical to the central flow axis of the bubbling area. The step portion is provided within the confines of a vertical extension of the above-mentioned cross-section to decrease the cross-sectional area of the bottom portion of the downcomer. The step platform has symmetrical outer edges which are spaced from the outer boundary edge of the cross-section of the downcomer to form a step defined bottom downcomer cross-section leading to a downcomer passageway defined in part by step wall portions of the downcomer. The configuration of the symmetrical stepwall panels of the bottom downcomer section at an elevation at or below the level of the step platform is preferably radially inwardly defined by the step platform""s outer edge (the step platform preferably has a central truncated chordal edge and two symmetrical outer end edges in conjunction with the downcomer""s outermost boundary). The two opposite end edges extend divergently outward from opposite ends of the step platform""s central edge and radially inward to the step platform""s inner most edge which coincides with the innermost boundary of the downcomer""s upper portion.
In larger trays, the vertical chordal step wall panel that encloses the lower central portion of the downcomer extends and is attached to the concave outer edge defining the downcomer""s outermost boundary, which in large trays is the tower""s interior, to efficiently add additional mechanical support. In connection with the foregoing features, the preferred embodiment of the invention also includes a multi-paneled wall extending below the downcomer step area which has a central chordal panel extending vertically downward from the central edge of the step platform and is located between the tiplet sections of the step wall. In large towers, this central panel in the step wall section is extended in length (either as a monolithic panel or by way of added extensions) and attached to the outermost boundary of the downcomer, which in a preferred embodiment is the tower""s interior, to provide additional mechanical support.
In the preferred embodiment, the lower downcomer cross-section has a central region defined by the central chordal panel of the step wall and the outermost boundary of the downcomer and has side regions defined by the tiplets of the step wall and the outermost boundary the downcomer. Each of these regions has a central region that is radially wider at or below the step platform plane than the outlet region extremities due to the concave shape of the outer downcomer boundary wall. These three areas combine to form the total cross-sectional area of the lower downcomer. The lower downcomer area at the bottom downcomer cross-section, has a total area no greater than 70% of the area of the top downcomer cross-section. The average radial distance between the central panel of the step wall and the outermost boundary of the downcomer is preferably no more than about 60% of the maximum transverse distance between the top downcomer inside wall and the outermost boundary of the downcomer.
The downcomer outlet opening comprises a continuous slot or series of slots formed by the vertical spacing between the lower edge of the downcomer step wall panels and the deck of the tray below, the width being perpendicular to the lower edge of the step wall panels and the length being parallel with respect to the lower edge of the step wall panels. A preferred outlet slot configuration has the slot locations restricted to the area formed by the gap between the step wall panels that are perpendicular to the central liquid flow axis and the deck of the tray below or between flow directing baffles below the tiplets and above the tray below in order to ensure that all liquid is released parallel to the central liquid flow axis. These flow directing baffles can be located and aligned with trusses in the tray below to also provide additional downcomer support in large vessels.
A preferred downcomer outlet slot opening has a chordal length as measured from the extreme opposite ends of the lower composite wall, which is at least 60% of the tower diameter. The downcomer outlet slot opening preferably extends along at least 40% of the full multi-chordal length of the downcomer step wall
A preferred embodiment of the invention utilizes a tray provided with strategically located directional jet tabs or push valves which are located and oriented in the active bubbling area to utilize the momentum of the vapor flow from the active bubbling area to accelerate movement of liquid from the liquid infeed area as necessary to provide uniform liquid flow onto and across the width of the bubbling area. Each of these directional apertures include an opening in the tray and a deflector for directing vapors ascending through the opening in a direction away from the liquid infeed area. The directional apertures have a spacing density which is less in areas near the central flow axis than in areas which are laterally spaced from the central flow axis. In a preferred embodiment of the invention, stationary deflectors are located above each of the tray apertures in the bubbling area. These deflectors have upstream and downstream ends connected to the tray so that vapors ascending through the tray apertures are introduced laterally into liquid in the bubbling area.
Preferably, the immediate liquid infeed area is substantially devoid of apertures to prevent ascending vapors from affecting the flow in a preceding downcomer of a tray thereabove and to prevent liquid in the preceding downcomer from weeping through the liquid infeed area. Since the downcomer configuration of this invention provides more effective liquid distribution to the active bubbling area than the prior art, it can be used with sieve tray decks with simple perforated active bubbling areas.
The chordal inner wall of the downcomer preferably has chordal extensions extending out into direct contact with, and attached to, vertical clamping bars at each end that are welded to the interior wall of the tower. The vertical portion of this chordal inner wall above the step platform has a channel beam form and principally supports the downcomer and the outlet end of the tray panels. The inner wall of the downcomer also includes vertical, intermediate wall sections that represent the extensions extending inward from contact with the tower at opposite chordal ends into the outer vertical side edging of the tiplets. The radially outer positioned tiplet panels are attached to the central vertical chordal panel of the step wall which, as noted above is preferably attached to vertical clamping bars at each end that are welded to the interior wall of the tower. Particularly for larger sized towers or vessels, there is further provided wing or chordal extensions (e.g., integral extremities of the central vertical chordal panel of the step outside the tiplet panels) that extend out into supportive relationship with the tower wall. The wing extensions are provided with liquid level equalization apertures. Extending inward from the step wall and forming the step platform are one or more horizontal panels that are supported by the chordal inner wall of the downcomer and/or the vertical panels of the step wall. Each downcomer, on a common tower side, is also preferably similarly situated (e.g., vertical tiplets falling along a common vertical plane, although different tiplet positioning potential is a very useful design feature of the present invention that permits different downcomer proportionment to adjust for liquid and vapor flow rates and physical properties along the vertical length of the tower).
In an embodiment of the invention, the downcomer step has an outer periphery that is defined by a plurality of linear sections, with three linear sections being the most preferred.